def updateVersionKnob():
node = nuke.thisNode()
knob = nuke.thisKnob()
# RUN ONLY IF THE TYPE KNOB CHANGES OR IF THE NODE PANEL IS OPENED
if not knob or knob.name() in [ 'versionType', 'showPanel' ]:
# GET THE VERSION DICTIONARY
versionDict = getVersions()
# POPULATE THE VERSION KNOB WITH THE VERSIONS REQUESTED THROUGH THE TYPE KNOB
node['_version'].setValues( versionDict[ node['versionType'].value() ] )
# SET THE A VALUE TO THE FIRST ITEM IN THE LIST
node['_version'].setValue(0)
```
因为createVersionKnobs和updateVersionKnob是回调函数，可以用nuke.thiNode() nuke.thisKnob()来引用对应的节点和knob。
在menu.py菜单中，当Read 节点创建时自动运行createVersionKnobs
```python
nuke.addOnUserCreate( assertManager.createVersionKnobs, nodeClass = 'Read' )